Full bridge converters use a switching circuit to generate a voltage, step a voltage up or down, or to isolate a voltage from a source. For example, a DC voltage may be input to the switching circuit. The switching circuit generates an AC signal based on the DC voltage to drive the primary side of a transformer (sometimes referred to herein simply as the “primary”). The transformer may step the voltage up or down. The AC voltage on the secondary side of the transformer (sometimes referred to herein simply as the “secondary”) may be used or rectified to a DC voltage.
In order to accurately control the output on the secondary side of the transformer, the voltage on the primary is adjusted. For example, the switching circuit may generate a pulse width modulated (PWM) signal that is used to drive the primary of the transformer. By adjusting the duty cycle of the PWM signal, the amplitude of output voltage on the secondary can be adjusted.
Although the PWM signal provides for very accurate adjustment of the primary, and thus, the secondary, the positive and negative duty cycles may not be equal. Therefore, current flowing in a first direction through the primary of the transformer may not be equal to the current flowing in a second and opposite direction. The result is the accumulation of a DC flux in the transformer, which can cause transformer saturation and circuit failure. Many embodiments of full bridge converters use DC blocking capacitors in series with the primary to block the DC voltage. The blocking capacitors are subject to failure, especially when they are subjected to high current flow in a power supply. The failure of a blocking capacitor can damage components connected to the full bridge converter.